This invention relates generally to an electro-optical cell for liquid crystal display of (electric) field effect type of improved characteristics such as visibility angle, contrast, and response.
There are two principal types of apparatus used as display apparatus for converting electrical signals into visible information by utilizing the optical anisotropy of a liquid crystal. In one of these, a dynamic scattering (DS) phenomenon wherein a nematic liquid crystal having negative dielectric anisotropy scatters light upon the application of an electric field is utilized for display (as disclosed, for example, in British Patent No. 1,167,486). In the other apparatus, a nematic liquid crystal having positive dielectric anisotropy is interposed and clamped between a pair of electrode base plates whose surfaces have been orientated thereby to impart optical rotation to the liquid crystal, and the optical rotation of the liquid crystal is caused to vary by the application of a suitable electric field and thus utilized for displaying (as disclosed, for example, in Japanese Patent Laid Open No. 11737/1972).
The latter apparatus of field effect (FE) type has such features as lower-voltage operation, lower power consumption, and longer life compared with the DS type of the former, and, for this reason, has wide utility in applications such as various displays and optical shades. A representative display apparatus of FE type has a construction, for example, as shown in exploded view in FIG. 1. The FE type electro-optical apparatus illustrated in FIG. 1 has an electro-optical cell 1 and a pair of polarizers 2 and 3 all disposed and aligned on a single optical axis.
The electro-optical cell 1 comprises a pair of glass plates 6 and 7, transparent electrodes 4 and 5 disposed on the inner sides of the glass plates 6 and 7, respectively, and a liquid crystal substance 8 having positive dielectric anisotropy and sandwiched between the electrodes 4 and 5. The electrode 4 has an inner surface which is orientation processed in the direction of the Z axis, for example as indicated, while the electrode 5 has an inner surface which is orientation processed in the Y-axis direction orthogonal to the Z axis.
When an electric field is not impressed between the electrodes 4 and 5, the longitudinal axes of the molecules of the liquid crystal substance 8 are aligned in the Z direction at the interface where the liquid crystal substance 8 contacts the electrode 4 and in the Y direction at the interface where the liquid crystal substance 8 contacts the electrode 5. Accordingly, the molecules of the liquid crystal substance 8 are arranged in a helical state wherein they are uniformly twisted through 90.degree. of angle between the electrodes 4 and 5.
Then, when the polarization directions of the polarizers 2 and 3 are respectively aligned in the Z and Y directions, and no electric field is applied between the electrodes 4 and 5, the light from a light source 9 passes through the polarizer 3 and the electro-optical cell 1, and is twisted to change its vibration plane by 90.degree. of angle while passing through the electro-optical cell 1, so that the light passes through the polarizer 3 to reach an observer 10.
When an electric field is impressed between the electrodes 4 and 5, the molecules of the liquid crystal substance 8 are so aligned that their dipoles are all directed toward the electric field. Since the molecular alignment of the liquid crystal substance 8 is thereby disarranged, the light from the light source 9, although passing through the polarizer 3 and the electro-optical cell 1, is shielded off by the polarizer 2. As a result, as viewed from the side of the observer 10, only a light of dark blue color resembling light transmitted through only the pair of polarizers 2 and 3 constituting mutually perpendicular nicol prisms is transmitted and is visible.
In an FE type electro-optical device of this transmission type, the electro-optical cell controls the vibration direction of the transmitted light depending on the presence or absence of an electric field and has the functional capability of causing or not causing the vibration plane of light passing through the polarizer on one side to coincide with the polarization direction of the polarizer disposed on the other side. In other words, a pair of polarizers is a valve which selects the vibration direction of light transmitted therethrough, while an electro-optical cell is a valve which controls the rotation of the vibration direction of transmitted light
In an FE type electro-optical device, as described above, the operational principle comprises controlling or realigning the initial orientation of the liquid crystal by means of an electric field and utilizing a change in the optical properties of the liquid crystal at this time. For this reason, the uniformity of the initial alignment (that is, the uniformity of the alignment at the time no electric field is applied) is especially important. As a measure for attaining uniformity of the initial alignment of a liquid crystal, the method of rubbing the electrode base plates in one direction each with a material such as cloth has heretofore been known. By this method, however, the alignment of the liquid crystal molecules differ from part to part, whereby the uniformity of the alignment is not adequate. Another problem is that the orientation is lost and the molecular alignment is destroyed in a short time.
Accordingly, as a measure to solve these problems, a method wherein a certain kind of surface-active agent is additionally used in addition to the rubbing of the electrode base plates in one direction (as disclosed, for example, in IBM Technical Disclosure Bulletin, Vol. 13, No. 11, Apr. 1971) is being used. However, while the uniformity of the alignment is improved to some extent by this method, the surface-active agent has inferior heat resistance, and, moreover, the surface-active agent is accompanied by the problem of its causing a deterioration of the liquid crystal. A further difficulty is that, as the application of the electric field is continued, the surface-active agent decomposes because of the electric field and undergoes degeneration, and the orientation is destroyed.
Furthermore, by the above described method wherein the electrode base plates in unprocessed state is rubbed in one direction, or that wherein a certain kind of surface-active agent is additionally used and rubbing is carried out in one direction, in the case where the major axes of the liquid crystal molecules near the electrode surfaces are respectively aligned parallelly to the electrode surfaces and are so orientated that the direction of these axes in one of the pair of electrodes is twisted by 90.degree. relative to those in the other electrode, the rotational direction of the liquid crystal molecules is not uniform, and left and right rotations exist in mixed state. Thus, there arises the problem of differences in the visibility angle, contrast, and response respectively in these two kinds of parts.
Accordingly, we have carried out research with the object of providing an electro-optical cell for FE type liquid crystal display having excellent display performance and not being accompanied by the above described problems when used in electronic desk-top calculators, various timepieces counter display panels, and the like. As a result, we have discovered that this object can be achieved by providing a poly-.alpha.-amino acid film, which has been orientation processed, on the surfaces containing electrodes of one pair of electrode base plates constituting parts of an electro-optical cell for FE type liquid crystal display. This invention has been developed on the basis of this discovery.
Throughout this disclosure, the term "poly-.alpha.-amino acid(s)" is intended to include condensation polymers of .alpha.-amino acids and their ester derivatives.